In recent years the demand for filamentary reinforced composites having improved mechanical properties has risen due to new applications in magnetic fusion energy, magnetohydrodynamics, and a variety of industrial and aerospace components. Epoxy resins are widely used in making these composites because they convert from a liquid or thermoplastic state to a chemically resistant thermoset solid state without the evolution of volatile materials. Also, epoxy resins exhibit the strength, adhesion, chemical resistance, and cure versatility that these composites require.
Epoxy resins are especially suitable for laminating applications where a "prepreg" or resin impregnated substrate of a fibrous material must be prepared. The epoxy resin in combination with curing agents and optional additives is usually dissolved in a suitable solvent, deposited or impregnated onto the substrate, and partially cured (B-staged) to produce a dry or, in some cases, a tacky resin impregnated substrate. The B-staged prepreg is then optionally stacked or individually laminated (fully cured) by application of heat and pressure.
Because epoxy resin systems are inherently brittle, it is difficult to produce composites having mechanical properties associated with "toughness" such as impact and crack resistance. While some toughened epoxy resins have been produced by modifying the epoxy resin with various low molecular weight carboxylic elastomers, the resulting composites still lack the impact strength desired for many modern high technology applications. The incorporation of higher molecular weight elastomers into an epoxy matrix has not been possible because the higher molecular weight elastomers are not readily soluble in the epoxy resins nor are the epoxy resins readily soluble in the elastomers. Also, because of thermodynamic considerations, it is not normally possible to dissolve both the epoxy resin and a high molecular weight elastomer in a common solvent.